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Date: Wed, 22 Jan 2020 23:38:51 +0100 From: Marco Elver <elver@...gle.com> To: Qian Cai <cai@....pw> Cc: Will Deacon <will@...nel.org>, mingo@...hat.com, peterz@...radead.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next On Wed, 22 Jan 2020, Qian Cai wrote: > > > > On Jan 22, 2020, at 11:59 AM, Will Deacon <will@...nel.org> wrote: > > > > I don't understand this; 'next' is a local variable. > > > > Not keen on the onslaught of random "add a READ_ONCE() to shut the > > sanitiser up" patches we're going to get from kcsan :( > > My fault. I suspect it is node->next. I’ll do a bit more testing to confirm. If possible, decode and get the line numbers. I have observed a data race in osq_lock before, however, this is the only one I have recently seen in osq_lock: read to 0xffff88812c12d3d4 of 4 bytes by task 23304 on cpu 0: osq_lock+0x170/0x2f0 kernel/locking/osq_lock.c:143 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(); } where static inline int node_cpu(struct optimistic_spin_node *node) { --> return node->cpu - 1; } write to 0xffff88812c12d3d4 of 4 bytes by task 23334 on cpu 1: osq_lock+0x89/0x2f0 kernel/locking/osq_lock.c:99 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; Thanks, -- Marco
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