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Date: Thu, 6 Jun 2019 21:42:33 +0800 From: Herbert Xu <herbert@...dor.apana.org.au> To: "Paul E. McKenney" <paulmck@...ux.ibm.com> Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org, torvalds@...ux-foundation.org Subject: Re: [PATCH RFC tip/core/rcu] Restore barrier() to rcu_read_lock() and rcu_read_unlock() On Thu, Jun 06, 2019 at 06:19:33AM -0700, Paul E. McKenney wrote: > Commit bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree > RCU readers") removed the barrier() calls from rcu_read_lock() and > rcu_write_lock() in CONFIG_PREEMPT=n&&CONFIG_PREEMPT_COUNT=n kernels. > Within RCU, this commit was OK, but it failed to account for things like > get_user() that can pagefault and that can be reordered by the compiler. > Lack of the barrier() calls in rcu_read_lock() and rcu_read_unlock() > can cause these page faults to migrate into RCU read-side critical > sections, which in CONFIG_PREEMPT=n kernels could result in too-short > grace periods and arbitrary misbehavior. Please see commit 386afc91144b > ("spinlocks and preemption points need to be at least compiler barriers") > for more details. > > This commit therefore restores the barrier() call to both rcu_read_lock() > and rcu_read_unlock(). It also removes them from places in the RCU update > machinery that used to need compensatory barrier() calls, effectively > reverting commit bb73c52bad36 ("rcu: Don't disable preemption for Tiny > and Tree RCU readers"). > > Reported-by: Herbert Xu <herbert@...dor.apana.org.au> > Reported-by: Linus Torvalds <torvalds@...ux-foundation.org> > Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com> Paul, Linus has already commited his patch: commit 66be4e66a7f422128748e3c3ef6ee72b20a6197b Author: Linus Torvalds <torvalds@...ux-foundation.org> Date: Mon Jun 3 13:26:20 2019 -0700 rcu: locking and unlocking need to always be at least barriers So you'll need to rebase this. Thanks, -- Email: Herbert Xu <herbert@...dor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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