linux-kernel - Re: [PATCH] rcu_sync: Fix comment to properly reflect rcu_sync

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Message-ID: <20220329232256.GE4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 29 Mar 2022 16:22:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     David Vernet <void@...ifault.com>
Cc:     frederic@...nel.org, jiangshanlai@...il.com,
        joel@...lfernandes.org, josh@...htriplett.org,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        quic_neeraju@...cinc.com, rcu@...r.kernel.org, rostedt@...dmis.org
Subject: Re: [PATCH] rcu_sync: Fix comment to properly reflect
 rcu_sync_exit() behavior

On Tue, Mar 29, 2022 at 03:26:13PM -0700, David Vernet wrote:
> rcu_sync_enter() is an rcu_sync API used by updaters which forces RCU
> readers (e.g. percpu-rwsem) to take a slow-path during an update. It does
> this by setting the gp_state of the rcu_sync object to GP_ENTER (i.e. > 0).
> In this state, readers will take a "slow path", such as having percpu-rwsem
> readers wait on a semaphore rather than just incrementing a reader count.
> When the updater has completed their work, they must invoke rcu_sync_exit()
> to signal to readers that they may again take their fastpaths.
> 
> rcu_sync_enter() currently has a comment that stipulates that a later call
> to rcu_sync_exit() (by an updater) will re-enable reader "slowpaths". This
> patch corrects the comment to properly reflect that rcu_sync_exit()
> re-enables reader fastpaths.
> 
> Signed-off-by: David Vernet <void@...ifault.com>

Good eyes, thank you!

As usual, I could not resist the urge to wordsmith.  Please take a look
at the following to see if I messed something up.

							Thanx, Paul

------------------------------------------------------------------------

commit b89e06a95c05009bcf31949814c42bc420f414a6
Author: David Vernet <void@...ifault.com>
Date:   Tue Mar 29 15:26:13 2022 -0700

    rcu_sync: Fix comment to properly reflect rcu_sync_exit() behavior
    
    The rcu_sync_enter() function is used by updaters to force RCU readers
    (e.g. percpu-rwsem) to use their slow paths during an update.  This is
    accomplished by setting the ->gp_state of the rcu_sync structure to
    GP_ENTER.  In the case of percpu-rwsem, the readers' slow path waits on
    a semaphore instead of just incrementing a reader count.  Each updater
    invokes the rcu_sync_exit() function to signal to readers that they
    may again take their fastpaths.  The rcu_sync_exit() function sets the
    ->gp_state of the rcu_sync structure to GP_EXIT, and if all goes well,
    after a grace period the ->gp_state reverts back to GP_IDLE.
    
    Unfortunately, the rcu_sync_enter() function currently has a comment
    incorrectly stating that rcu_sync_exit() (by an updater) will re-enable
    reader "slowpaths".  This patch changes the comment to state that this
    function re-enables reader fastpaths.
    
    Signed-off-by: David Vernet <void@...ifault.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 33d896d85902..5cefc702158f 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -111,7 +111,7 @@ static void rcu_sync_func(struct rcu_head *rhp)
  * a slowpath during the update.  After this function returns, all
  * subsequent calls to rcu_sync_is_idle() will return false, which
  * tells readers to stay off their fastpaths.  A later call to
- * rcu_sync_exit() re-enables reader slowpaths.
+ * rcu_sync_exit() re-enables reader fastpaths.
  *
  * When called in isolation, rcu_sync_enter() must wait for a grace
  * period, however, closely spaced calls to rcu_sync_enter() can