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Diffstat (limited to 'mysys/lf_alloc-pin.c')
| -rw-r--r-- | mysys/lf_alloc-pin.c | 535 |
1 files changed, 535 insertions, 0 deletions
diff --git a/mysys/lf_alloc-pin.c b/mysys/lf_alloc-pin.c new file mode 100644 index 00000000000..7c3e3785b68 --- /dev/null +++ b/mysys/lf_alloc-pin.c @@ -0,0 +1,535 @@ +/* QQ: TODO multi-pinbox */ +/* Copyright (C) 2006 MySQL AB + + This program is free software; you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 2 of the License, or + (at your option) any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program; if not, write to the Free Software + Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ + +/* + wait-free concurrent allocator based on pinning addresses + + It works as follows: every thread (strictly speaking - every CPU, but + it's too difficult to do) has a small array of pointers. They're called + "pins". Before using an object its address must be stored in this array + (pinned). When an object is no longer necessary its address must be + removed from this array (unpinned). When a thread wants to free() an + object it scans all pins of all threads to see if somebody has this + object pinned. If yes - the object is not freed (but stored in a + "purgatory"). To reduce the cost of a single free() pins are not scanned + on every free() but only added to (thread-local) purgatory. On every + LF_PURGATORY_SIZE free() purgatory is scanned and all unpinned objects + are freed. + + Pins are used to solve ABA problem. To use pins one must obey + a pinning protocol: + + 1. Let's assume that PTR is a shared pointer to an object. Shared means + that any thread may modify it anytime to point to a different object + and free the old object. Later the freed object may be potentially + allocated by another thread. If we're unlucky that other thread may + set PTR to point to this object again. This is ABA problem. + 2. Create a local pointer LOCAL_PTR. + 3. Pin the PTR in a loop: + do + { + LOCAL_PTR= PTR; + pin(PTR, PIN_NUMBER); + } while (LOCAL_PTR != PTR) + 4. It is guaranteed that after the loop has ended, LOCAL_PTR + points to an object (or NULL, if PTR may be NULL), that + will never be freed. It is not guaranteed though + that LOCAL_PTR == PTR (as PTR can change any time) + 5. When done working with the object, remove the pin: + unpin(PIN_NUMBER) + 6. When copying pins (as in the list traversing loop: + pin(CUR, 1); + while () + { + do // standard + { // pinning + NEXT=CUR->next; // loop + pin(NEXT, 0); // see #3 + } while (NEXT != CUR->next); // above + ... + ... + CUR=NEXT; + pin(CUR, 1); // copy pin[0] to pin[1] + } + which keeps CUR address constantly pinned), note than pins may be + copied only upwards (!!!), that is pin[N] to pin[M], M > N. + 7. Don't keep the object pinned longer than necessary - the number of + pins you have is limited (and small), keeping an object pinned + prevents its reuse and cause unnecessary mallocs. + + Explanations: + + 3. The loop is important. The following can occur: + thread1> LOCAL_PTR= PTR + thread2> free(PTR); PTR=0; + thread1> pin(PTR, PIN_NUMBER); + now thread1 cannot access LOCAL_PTR, even if it's pinned, + because it points to a freed memory. That is, it *must* + verify that it has indeed pinned PTR, the shared pointer. + + 6. When a thread wants to free some LOCAL_PTR, and it scans + all lists of pins to see whether it's pinned, it does it + upwards, from low pin numbers to high. Thus another thread + must copy an address from one pin to another in the same + direction - upwards, otherwise the scanning thread may + miss it. + + Implementation details: + + Pins are given away from a "pinbox". Pinbox is stack-based allocator. + It used dynarray for storing pins, new elements are allocated by dynarray + as necessary, old are pushed in the stack for reuse. ABA is solved by + versioning a pointer - because we use an array, a pointer to pins is 16 bit, + upper 16 bits are used for a version. + + It is assumed that pins belong to a THD and are not transferable + between THD's (LF_PINS::stack_ends_here being a primary reason + for this limitation). +*/ +#include <my_global.h> +#include <my_sys.h> +#include <lf.h> + +#define LF_PINBOX_MAX_PINS 65536 + +static void _lf_pinbox_real_free(LF_PINS *pins); + +/* + Initialize a pinbox. Normally called from lf_alloc_init. + See the latter for details. +*/ +void lf_pinbox_init(LF_PINBOX *pinbox, uint free_ptr_offset, + lf_pinbox_free_func *free_func, void *free_func_arg) +{ + DBUG_ASSERT(free_ptr_offset % sizeof(void *) == 0); + compile_time_assert(sizeof(LF_PINS) == 128); + lf_dynarray_init(&pinbox->pinarray, sizeof(LF_PINS)); + pinbox->pinstack_top_ver= 0; + pinbox->pins_in_array= 0; + pinbox->free_ptr_offset= free_ptr_offset; + pinbox->free_func= free_func; + pinbox->free_func_arg= free_func_arg; +} + +void lf_pinbox_destroy(LF_PINBOX *pinbox) +{ + lf_dynarray_destroy(&pinbox->pinarray); +} + +/* + Get pins from a pinbox. Usually called via lf_alloc_get_pins() or + lf_hash_get_pins(). + + SYNOPSYS + pinbox - + + DESCRIPTION + get a new LF_PINS structure from a stack of unused pins, + or allocate a new one out of dynarray. + + NOTE + It is assumed that pins belong to a thread and are not transferable + between threads. +*/ +LF_PINS *_lf_pinbox_get_pins(LF_PINBOX *pinbox) +{ + uint32 pins, next, top_ver; + LF_PINS *el; + /* + We have an array of max. 64k elements. + The highest index currently allocated is pinbox->pins_in_array. + Freed elements are in a lifo stack, pinstack_top_ver. + pinstack_top_ver is 32 bits; 16 low bits are the index in the + array, to the first element of the list. 16 high bits are a version + (every time the 16 low bits are updated, the 16 high bits are + incremented). Versioniong prevents the ABA problem. + */ + top_ver= pinbox->pinstack_top_ver; + do + { + if (!(pins= top_ver % LF_PINBOX_MAX_PINS)) + { + /* the stack of free elements is empty */ + pins= my_atomic_add32((int32 volatile*) &pinbox->pins_in_array, 1)+1; + if (unlikely(pins >= LF_PINBOX_MAX_PINS)) + return 0; + /* + note that the first allocated element has index 1 (pins==1). + index 0 is reserved to mean "NULL pointer" + */ + el= (LF_PINS *)_lf_dynarray_lvalue(&pinbox->pinarray, pins); + if (unlikely(!el)) + return 0; + break; + } + el= (LF_PINS *)_lf_dynarray_value(&pinbox->pinarray, pins); + next= el->link; + } while (!my_atomic_cas32((int32 volatile*) &pinbox->pinstack_top_ver, + (int32*) &top_ver, + top_ver-pins+next+LF_PINBOX_MAX_PINS)); + /* + set el->link to the index of el in the dynarray (el->link has two usages: + - if element is allocated, it's its own index + - if element is free, it's its next element in the free stack + */ + el->link= pins; + el->purgatory_count= 0; + el->pinbox= pinbox; + el->stack_ends_here= & my_thread_var->stack_ends_here; + return el; +} + +/* + Put pins back to a pinbox. Usually called via lf_alloc_put_pins() or + lf_hash_put_pins(). + + DESCRIPTION + empty the purgatory (XXX deadlock warning below!), + push LF_PINS structure to a stack +*/ +void _lf_pinbox_put_pins(LF_PINS *pins) +{ + LF_PINBOX *pinbox= pins->pinbox; + uint32 top_ver, nr; + nr= pins->link; +#ifdef MY_LF_EXTRA_DEBUG + { + int i; + for (i= 0; i < LF_PINBOX_PINS; i++) + DBUG_ASSERT(pins->pin[i] == 0); + } +#endif + /* + XXX this will deadlock if other threads will wait for + the caller to do something after _lf_pinbox_put_pins(), + and they would have pinned addresses that the caller wants to free. + Thus: only free pins when all work is done and nobody can wait for you!!! + */ + while (pins->purgatory_count) + { + _lf_pinbox_real_free(pins); + if (pins->purgatory_count) + { + my_atomic_rwlock_wrunlock(&pins->pinbox->pinarray.lock); + pthread_yield(); + my_atomic_rwlock_wrlock(&pins->pinbox->pinarray.lock); + } + } + top_ver= pinbox->pinstack_top_ver; + do + { + pins->link= top_ver % LF_PINBOX_MAX_PINS; + } while (!my_atomic_cas32((int32 volatile*) &pinbox->pinstack_top_ver, + (int32*) &top_ver, + top_ver-pins->link+nr+LF_PINBOX_MAX_PINS)); + return; +} + +static int ptr_cmp(void **a, void **b) +{ + return *a < *b ? -1 : *a == *b ? 0 : 1; +} + +#define add_to_purgatory(PINS, ADDR) \ + do \ + { \ + *(void **)((char *)(ADDR)+(PINS)->pinbox->free_ptr_offset)= \ + (PINS)->purgatory; \ + (PINS)->purgatory= (ADDR); \ + (PINS)->purgatory_count++; \ + } while (0) + +/* + Free an object allocated via pinbox allocator + + DESCRIPTION + add an object to purgatory. if necessary, call _lf_pinbox_real_free() + to actually free something. +*/ +void _lf_pinbox_free(LF_PINS *pins, void *addr) +{ + add_to_purgatory(pins, addr); + if (pins->purgatory_count % LF_PURGATORY_SIZE) + _lf_pinbox_real_free(pins); +} + +struct st_harvester { + void **granary; + int npins; +}; + +/* + callback for _lf_dynarray_iterate: + scan all pins of all threads and accumulate all pins +*/ +static int harvest_pins(LF_PINS *el, struct st_harvester *hv) +{ + int i; + LF_PINS *el_end= el+min(hv->npins, LF_DYNARRAY_LEVEL_LENGTH); + for (; el < el_end; el++) + { + for (i= 0; i < LF_PINBOX_PINS; i++) + { + void *p= el->pin[i]; + if (p) + *hv->granary++= p; + } + } + /* + hv->npins may become negative below, but it means that + we're on the last dynarray page and harvest_pins() won't be + called again. We don't bother to make hv->npins() correct + (that is 0) in this case. + */ + hv->npins-= LF_DYNARRAY_LEVEL_LENGTH; + return 0; +} + +/* + callback for _lf_dynarray_iterate: + scan all pins of all threads and see if addr is present there +*/ +static int match_pins(LF_PINS *el, void *addr) +{ + int i; + LF_PINS *el_end= el+LF_DYNARRAY_LEVEL_LENGTH; + for (; el < el_end; el++) + for (i= 0; i < LF_PINBOX_PINS; i++) + if (el->pin[i] == addr) + return 1; + return 0; +} + +#if STACK_DIRECTION < 0 +#define available_stack_size(CUR,END) (long) ((char*)(CUR) - (char*)(END)) +#else +#define available_stack_size(CUR,END) (long) ((char*)(END) - (char*)(CUR)) +#endif + +#define next_node(P, X) (*((uchar * volatile *)(((uchar *)(X)) + (P)->free_ptr_offset))) +#define anext_node(X) next_node(&allocator->pinbox, (X)) + +/* + Scan the purgatory and free everything that can be freed +*/ +static void _lf_pinbox_real_free(LF_PINS *pins) +{ + int npins, alloca_size; + void *list, **addr; + uchar *first, *last= NULL; + LF_PINBOX *pinbox= pins->pinbox; + + LINT_INIT(first); + npins= pinbox->pins_in_array+1; + +#ifdef HAVE_ALLOCA + alloca_size= sizeof(void *)*LF_PINBOX_PINS*npins; + /* create a sorted list of pinned addresses, to speed up searches */ + if (available_stack_size(&pinbox, *pins->stack_ends_here) > alloca_size) + { + struct st_harvester hv; + addr= (void **) alloca(alloca_size); + hv.granary= addr; + hv.npins= npins; + /* scan the dynarray and accumulate all pinned addresses */ + _lf_dynarray_iterate(&pinbox->pinarray, + (lf_dynarray_func)harvest_pins, &hv); + + npins= hv.granary-addr; + /* and sort them */ + if (npins) + qsort(addr, npins, sizeof(void *), (qsort_cmp)ptr_cmp); + } + else +#endif + addr= 0; + + list= pins->purgatory; + pins->purgatory= 0; + pins->purgatory_count= 0; + while (list) + { + void *cur= list; + list= *(void **)((char *)cur+pinbox->free_ptr_offset); + if (npins) + { + if (addr) /* use binary search */ + { + void **a, **b, **c; + for (a= addr, b= addr+npins-1, c= a+(b-a)/2; (b-a) > 1; c= a+(b-a)/2) + if (cur == *c) + a= b= c; + else if (cur > *c) + a= c; + else + b= c; + if (cur == *a || cur == *b) + goto found; + } + else /* no alloca - no cookie. linear search here */ + { + if (_lf_dynarray_iterate(&pinbox->pinarray, + (lf_dynarray_func)match_pins, cur)) + goto found; + } + } + /* not pinned - freeing */ + if (last) + last= next_node(pinbox, last)= (uchar *)cur; + else + first= last= (uchar *)cur; + continue; +found: + /* pinned - keeping */ + add_to_purgatory(pins, cur); + } + if (last) + pinbox->free_func(first, last, pinbox->free_func_arg); +} + +/* lock-free memory allocator for fixed-size objects */ + +LF_REQUIRE_PINS(1) + +/* + callback for _lf_pinbox_real_free to free a list of unpinned objects - + add it back to the allocator stack + + DESCRIPTION + 'first' and 'last' are the ends of the linked list of nodes: + first->el->el->....->el->last. Use first==last to free only one element. +*/ +static void alloc_free(uchar *first, + uchar volatile *last, + LF_ALLOCATOR *allocator) +{ + /* + we need a union here to access type-punned pointer reliably. + otherwise gcc -fstrict-aliasing will not see 'tmp' changed in the loop + */ + union { uchar * node; void *ptr; } tmp; + tmp.node= allocator->top; + do + { + anext_node(last)= tmp.node; + } while (!my_atomic_casptr((void **)(char *)&allocator->top, + (void **)&tmp.ptr, first) && LF_BACKOFF); +} + +/* + initialize lock-free allocator + + SYNOPSYS + allocator - + size a size of an object to allocate + free_ptr_offset an offset inside the object to a sizeof(void *) + memory that is guaranteed to be unused after + the object is put in the purgatory. Unused by ANY + thread, not only the purgatory owner. + This memory will be used to link waiting-to-be-freed + objects in a purgatory list. +*/ +void lf_alloc_init(LF_ALLOCATOR *allocator, uint size, uint free_ptr_offset) +{ + lf_pinbox_init(&allocator->pinbox, free_ptr_offset, + (lf_pinbox_free_func *)alloc_free, allocator); + allocator->top= 0; + allocator->mallocs= 0; + allocator->element_size= size; + allocator->constructor= 0; + allocator->destructor= 0; + DBUG_ASSERT(size >= sizeof(void*) + free_ptr_offset); +} + +/* + destroy the allocator, free everything that's in it + + NOTE + As every other init/destroy function here and elsewhere it + is not thread safe. No, this function is no different, ensure + that no thread needs the allocator before destroying it. + We are not responsible for any damage that may be caused by + accessing the allocator when it is being or has been destroyed. + Oh yes, and don't put your cat in a microwave. +*/ +void lf_alloc_destroy(LF_ALLOCATOR *allocator) +{ + uchar *node= allocator->top; + while (node) + { + uchar *tmp= anext_node(node); + if (allocator->destructor) + allocator->destructor(node); + my_free((void *)node, MYF(0)); + node= tmp; + } + lf_pinbox_destroy(&allocator->pinbox); + allocator->top= 0; +} + +/* + Allocate and return an new object. + + DESCRIPTION + Pop an unused object from the stack or malloc it is the stack is empty. + pin[0] is used, it's removed on return. +*/ +void *_lf_alloc_new(LF_PINS *pins) +{ + LF_ALLOCATOR *allocator= (LF_ALLOCATOR *)(pins->pinbox->free_func_arg); + uchar *node; + for (;;) + { + do + { + node= allocator->top; + _lf_pin(pins, 0, node); + } while (node != allocator->top && LF_BACKOFF); + if (!node) + { + node= (void *)my_malloc(allocator->element_size, MYF(MY_WME)); + if (allocator->constructor) + allocator->constructor(node); +#ifdef MY_LF_EXTRA_DEBUG + if (likely(node != 0)) + my_atomic_add32(&allocator->mallocs, 1); +#endif + break; + } + if (my_atomic_casptr((void **)(char *)&allocator->top, + (void *)&node, anext_node(node))) + break; + } + _lf_unpin(pins, 0); + return node; +} + +/* + count the number of objects in a pool. + + NOTE + This is NOT thread-safe !!! +*/ +uint lf_alloc_pool_count(LF_ALLOCATOR *allocator) +{ + uint i; + uchar *node; + for (node= allocator->top, i= 0; node; node= anext_node(node), i++) + /* no op */; + return i; +} + |