From: Suresh Siddha Date: Mon, 2 Jun 2008 22:57:27 +0000 (-0700) Subject: x86, fpu: fix CONFIG_PREEMPT=y corruption of application's FPU stack X-Git-Tag: v2.6.26-rc5~5^2 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=870568b39064cab2dd971fe57969916036982862;p=pandora-kernel.git x86, fpu: fix CONFIG_PREEMPT=y corruption of application's FPU stack Jürgen Mell reported an FPU state corruption bug under CONFIG_PREEMPT, and bisected it to commit v2.6.19-1363-gacc2076, "i386: add sleazy FPU optimization". Add tsk_used_math() checks to prevent calling math_state_restore() which can sleep in the case of !tsk_used_math(). This prevents making a blocking call in __switch_to(). Apparently "fpu_counter > 5" check is not enough, as in some signal handling and fork/exec scenarios, fpu_counter > 5 and !tsk_used_math() is possible. It's a side effect though. This is the failing scenario: process 'A' in save_i387_ia32() just after clear_used_math() Got an interrupt and pre-empted out. At the next context switch to process 'A' again, kernel tries to restore the math state proactively and sees a fpu_counter > 0 and !tsk_used_math() This results in init_fpu() during the __switch_to()'s math_state_restore() And resulting in fpu corruption which will be saved/restored (save_i387_fxsave and restore_i387_fxsave) during the remaining part of the signal handling after the context switch. Bisected-by: Jürgen Mell Signed-off-by: Suresh Siddha Tested-by: Jürgen Mell Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner Cc: stable@kernel.org --- Reading git-diff-tree failed