From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTP id C4486DDED5 for ; Fri, 19 Oct 2007 14:11:43 +1000 (EST) Subject: Re: [PATCH] synchronize_irq needs a barrier From: Benjamin Herrenschmidt To: Linus Torvalds In-Reply-To: References: <1192745137.7367.40.camel@pasglop> <1192749449.7367.51.camel@pasglop> <20071019023219.GB8453@gondor.apana.org.au> Content-Type: text/plain Date: Fri, 19 Oct 2007 14:11:13 +1000 Message-Id: <1192767073.7367.84.camel@pasglop> Mime-Version: 1.0 Cc: Herbert Xu , Linux Kernel Mailing List , linuxppc-dev@ozlabs.org, Thomas Gleixner , akpm@linux-foundation.org, Ingo Molnar Reply-To: benh@kernel.crashing.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , > > repeat: > /* Optimistic, no-locking loop */ > while (desc->status & IRQ_INPROGRESS) > cpu_relax(); > > /* Ok, that indicated we're done: double-check carefully */ > spin_lock_irqsave(&desc->lock, flags); > status = desc->status; > spin_unlock_irqrestore(&desc->lock, flags); > > /* Oops, that failed? */ > if (status & IRQ_INPROGRESS) > goto repeat; > > Hmm? Paulus and I convinced ourselves that this would work. If we call our variable that gets set before synchronize_irq and read in the IRQ handler "foo", we get to: - writing foo can travel down at most to just before the unlock in the code above - reading foo can travel up out of the IRQ handler at most just after the lock in the code that sets IRQ_INPROGRESS. The whole lock/set IRQ_INPROGRESS/unlock path can then only happen before the locked section above, in which case we see and wait nicely and all is good, or after, in which case the store to foo will be visible to the IRQ handler as it will be ordered with the unlock in the code above. Pfiew ! So Acked-by: Benjamin Herrenschmidt Thanks ! Ben.