Commit-ID: 7e5a2c1729f1612618ed236249a15bf15f309325 Gitweb: http://git.kernel.org/tip/7e5a2c1729f1612618ed236249a15bf15f309325 Author: Jason Low <[email protected]> AuthorDate: Thu, 30 Apr 2015 17:28:14 -0700 Committer: Ingo Molnar <[email protected]> CommitDate: Fri, 8 May 2015 12:13:13 +0200
sched/numa: Document usages of mm->numa_scan_seq The p->mm->numa_scan_seq is accessed using READ_ONCE/WRITE_ONCE and modified without exclusive access. It is not clear why it is accessed this way. This patch provides some documentation on that. Suggested-by: Ingo Molnar <[email protected]> Signed-off-by: Jason Low <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Acked-by: Thomas Gleixner <[email protected]> Acked-by: Rik van Riel <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Aswin Chandramouleeswaran <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Davidlohr Bueso <[email protected]> Cc: Frederic Weisbecker <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Mike Galbraith <[email protected]> Cc: Oleg Nesterov <[email protected]> Cc: Paul E. McKenney <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Preeti U Murthy <[email protected]> Cc: Scott J Norton <[email protected]> Cc: Steven Rostedt <[email protected]> Cc: Waiman Long <[email protected]> Link: http://lkml.kernel.org/r/1430440094.2475.61.camel@j-VirtualBox Signed-off-by: Ingo Molnar <[email protected]> --- kernel/sched/fair.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d6915a0..f18ddb7 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1794,6 +1794,11 @@ static void task_numa_placement(struct task_struct *p) u64 runtime, period; spinlock_t *group_lock = NULL; + /* + * The p->mm->numa_scan_seq field gets updated without + * exclusive access. Use READ_ONCE() here to ensure + * that the field is read in a single access: + */ seq = READ_ONCE(p->mm->numa_scan_seq); if (p->numa_scan_seq == seq) return; @@ -2107,6 +2112,14 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) static void reset_ptenuma_scan(struct task_struct *p) { + /* + * We only did a read acquisition of the mmap sem, so + * p->mm->numa_scan_seq is written to without exclusive access + * and the update is not guaranteed to be atomic. That's not + * much of an issue though, since this is just used for + * statistical sampling. Use READ_ONCE/WRITE_ONCE, which are not + * expensive, to avoid any form of compiler optimizations: + */ WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); p->mm->numa_scan_offset = 0; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
