The spinlock in eventfd_poll is trying to protect the count of events so it can decide if it should return POLLIN, POLLERR, or POLLOUT. But, because of the way we drop the lock after calling poll_wait, and drop it again before returning, we have the same pile of races with the lock as we do with a single read of ctx->count(). This replaces the lock with a read barrier and single read. eventfd_write does a single bump of ctx->count, so this should not add new races with adding events. eventfd_read is similar, it will do a single decrement with the lock held, and so we're making the race with concurrent readers slightly larger. This spinlock is the top CPU user in kernel code during one of our workloads. Removing it gives us a ~2% boost. Signed-off-by: Chris Mason <clm@xxxxxx> --- fs/eventfd.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index d6a88e7..cdbdb96 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -119,17 +119,18 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait) struct eventfd_ctx *ctx = file->private_data; unsigned int events = 0; unsigned long flags; + unsigned int count; poll_wait(file, &ctx->wqh, wait); + smp_rmb(); + count = ctx->count; - spin_lock_irqsave(&ctx->wqh.lock, flags); - if (ctx->count > 0) + if (count > 0) events |= POLLIN; - if (ctx->count == ULLONG_MAX) + if (count == ULLONG_MAX) events |= POLLERR; - if (ULLONG_MAX - 1 > ctx->count) + if (ULLONG_MAX - 1 > count) events |= POLLOUT; - spin_unlock_irqrestore(&ctx->wqh.lock, flags); return events; } -- 1.8.1 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
