#include #include #include /* * An MCS like lock especially tailored for optimistic spinning for sleeping * lock implementations (mutex, rwsem, etc). * * Using a single mcs node per CPU is safe because sleeping locks should not be * called from interrupt context and we have preemption disabled while * spinning. */ static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); /* * We use the value 0 to represent "no CPU", thus the encoded value * will be the CPU number incremented by 1. */ static inline int encode_cpu(int cpu_nr) { return cpu_nr + 1; } static inline int node_cpu(struct optimistic_spin_node *node) { return node->cpu - 1; } static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) { int cpu_nr = encoded_cpu_val - 1; return per_cpu_ptr(&osq_node, cpu_nr); } /* * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. * Can return NULL in case we were the last queued and we updated @lock instead. */ static inline struct optimistic_spin_node * osq_wait_next(struct optimistic_spin_queue *lock, struct optimistic_spin_node *node, struct optimistic_spin_node *prev) { struct optimistic_spin_node *next = NULL; int curr = encode_cpu(smp_processor_id()); int old; /* * If there is a prev node in queue, then the 'old' value will be * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if * we're currently last in queue, then the queue will then become empty. */ old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; for (;;) { if (atomic_read(&lock->tail) == curr && atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) { /* * We were the last queued, we moved @lock back. @prev * will now observe @lock and will complete its * unlock()/unqueue(). */ break; } /* * We must xchg() the @node->next value, because if we were to * leave it in, a concurrent unlock()/unqueue() from * @node->next might complete Step-A and think its @prev is * still valid. * * If the concurrent unlock()/unqueue() wins the race, we'll * wait for either @lock to point to us, through its Step-B, or * wait for a new @node->next from its Step-C. */ if (node->next) { next = xchg(&node->next, NULL); if (next) break; } cpu_relax(); } return next; } bool osq_lock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); struct optimistic_spin_node *prev, *next; int curr = encode_cpu(smp_processor_id()); int old; node->locked = 0; node->next = NULL; node->cpu = curr; /* * We need both ACQUIRE (pairs with corresponding RELEASE in * unlock() uncontended, or fastpath) and RELEASE (to publish * the node fields we just initialised) semantics when updating * the lock tail. */ old = atomic_xchg(&lock->tail, curr); if (old == OSQ_UNLOCKED_VAL) return true; prev = decode_cpu(old); node->prev = prev; WRITE_ONCE(prev->next, node); /* * Normally @prev is untouchable after the above store; because at that * moment unlock can proceed and wipe the node element from stack. * * However, since our nodes are static per-cpu storage, we're * guaranteed their existence -- this allows us to apply * cmpxchg in an attempt to undo our queueing. */ while (!READ_ONCE(node->locked)) { /* * If we need to reschedule bail... so we can block. * Use vcpu_is_preempted() to avoid waiting for a preempted * lock holder: */ if (need_resched() || vcpu_is_preempted(node_cpu(node->prev))) goto unqueue; cpu_relax(); } return true; unqueue: /* * Step - A -- stabilize @prev * * Undo our @prev->next assignment; this will make @prev's * unlock()/unqueue() wait for a next pointer since @lock points to us * (or later). */ for (;;) { if (prev->next == node && cmpxchg(&prev->next, node, NULL) == node) break; /* * We can only fail the cmpxchg() racing against an unlock(), * in which case we should observe @node->locked becomming * true. */ if (smp_load_acquire(&node->locked)) return true; cpu_relax(); /* * Or we race against a concurrent unqueue()'s step-B, in which * case its step-C will write us a new @node->prev pointer. */ prev = READ_ONCE(node->prev); } /* * Step - B -- stabilize @next * * Similar to unlock(), wait for @node->next or move @lock from @node * back to @prev. */ next = osq_wait_next(lock, node, prev); if (!next) return false; /* * Step - C -- unlink * * @prev is stable because its still waiting for a new @prev->next * pointer, @next is stable because our @node->next pointer is NULL and * it will wait in Step-A. */ WRITE_ONCE(next->prev, prev); WRITE_ONCE(prev->next, next); return false; } void osq_unlock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node, *next; int curr = encode_cpu(smp_processor_id()); /* * Fast path for the uncontended case. */ if (likely(atomic_cmpxchg_release(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) return; /* * Second most likely case. */ node = this_cpu_ptr(&osq_node); next = xchg(&node->next, NULL); if (next) { WRITE_ONCE(next->locked, 1); return; } next = osq_wait_next(lock, node, NULL); if (next) WRITE_ONCE(next->locked, 1); } /thin-provisioning.txt?id=e7aa8c2eb11ba69b1b69099c3c7bd6be3087b0ba'>patch) treef63906f41699c8e38af9d12b063e2ceab0286ef2 /Documentation/device-mapper/thin-provisioning.txt parente34bac726d27056081d0250c0e173e4b155aa340 (diff)parent868c97a846a73e937d835b09b8c885a69df50ec8 (diff)
Merge tag 'docs-4.10' of git://git.lwn.net/linuxHEADmaster
Pull documentation update from Jonathan Corbet: "These are the documentation changes for 4.10. It's another busy cycle for the docs tree, as the sphinx conversion continues. Highlights include: - Further work on PDF output, which remains a bit of a pain but should be more solid now. - Five more DocBook template files converted to Sphinx. Only 27 to go... Lots of plain-text files have also been converted and integrated. - Images in binary formats have been replaced with more source-friendly versions. - Various bits of organizational work, including the renaming of various files discussed at the kernel summit. - New documentation for the device_link mechanism. ... and, of course, lots of typo fixes and small updates" * tag 'docs-4.10' of git://git.lwn.net/linux: (193 commits) dma-buf: Extract dma-buf.rst Update Documentation/00-INDEX docs: 00-INDEX: document directories/files with no docs docs: 00-INDEX: remove non-existing entries docs: 00-INDEX: add missing entries for documentation files/dirs docs: 00-INDEX: consolidate process/ and admin-guide/ description scripts: add a script to check if Documentation/00-INDEX is sane Docs: change sh -> awk in REPORTING-BUGS Documentation/core-api/device_link: Add initial documentation core-api: remove an unexpected unident ppc/idle: Add documentation for powersave=off Doc: Correct typo, "Introdution" => "Introduction" Documentation/atomic_ops.txt: convert to ReST markup Documentation/local_ops.txt: convert to ReST markup Documentation/assoc_array.txt: convert to ReST markup docs-rst: parse-headers.pl: cleanup the documentation docs-rst: fix media cleandocs target docs-rst: media/Makefile: reorganize the rules docs-rst: media: build SVG from graphviz files docs-rst: replace bayer.png by a SVG image ...
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