linux-kernel - One potential issue with concurrent execution of RCU callbacks...

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Message-ID: <20201208145810.GA4875@paulmck-ThinkPad-P72>
Date:   Tue, 8 Dec 2020 06:58:10 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     frederic@...nel.org
Cc:     boqun.feng@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: One potential issue with concurrent execution of RCU callbacks...

Hello, Frederic,

Boqun just asked if RCU callbacks ran in BH-disabled context to avoid
concurrent execution of the same callback.  Of course, this raises the
question of whether a self-posting callback can have two instances of
itself running concurrently while a CPU is in the process of transitioning
between softirq and rcuo invocation of callbacks.

I believe that the answer is "no" because BH-disabled context is
an implicit RCU read-side critical section.  Therefore, the initial
invocation of the RCU callback must complete in order for a new grace
period to complete, and a new grace period must complete before the
second invocation of that same callback to start.

Does that make sense, or am I missing something?

							Thanx, Paul