linux-kernel - Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 21 Nov 2006 12:56:21 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Oleg Nesterov <oleg@...sign.ru>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

Here's another potential problem with the fast path approach.  It's not 
very serious, but you might want to keep it in mind.

The idea is that a reader can start up on one CPU and finish on another, 
and a writer might see the finish event but not the start event.  For 
example:

	Reader A enters the critical section on CPU 0 and starts
	accessing the old data area.

	Writer B updates the data pointer and starts executing
	srcu_readers_active_idx() to check if the fast path can be
	used.  It sees per_cpu_ptr(0)->c[idx] == 1 because of
	Reader A.

	Reader C runs srcu_read_lock() on CPU 0, setting 
	per_cpu_ptr[0]->c[idx] to 2.

	Reader C migrates to CPU 1 and leaves the critical section;
	srcu_read_unlock() sets per_cpu_ptr(1)->c[idx] to -1.

	Writer B finishes the cpu loop in srcu_readers_active_idx(),
	seeing per_cpu_ptr(1)->c[idx] == -1.  It computes sum =
	1 + -1 == 0, takes the fast path, and exits immediately
	from synchronize_srcu().

	Writer B deallocates the old data area while Reader A is still
	using it.

This requires two context switches to take place while the cpu loop in
srcu_readers_active_idx() runs, so perhaps it isn't realistic.  Is it
worth worrying about?

Alan

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/