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Subject[patch] irqlock patch -G3. [was Re: odd memory corruption in 2.5.27?]

On Tue, 23 Jul 2002, george anzinger wrote:

> I just spent a month tracking down this issue. It comes
> down to the slab allocater using per cpu data structures and
> protecting them with a combination of interrupt disables and
> spin_locks. Preemption is allowed (incorrectly) if
> interrupts are off and preempt_count goes to zero on the
> spin_unlock. [...]

> The proposed fix is to catch the attempted preemption in
> preempt_schedule() and just return if the interrupt system
> is off. [...]

this is most definitely not the correct fix ...

i'm quite convinced that the fix is to avoid illegal preemption, not to
work it around.

i've written debugging code that caught and reported this slab.c bug
within minutes. The code detects the irqs-off condition in schedule(). You
can find it my latest irqlock patchset, at:

http://redhat.com/~mingo/remove-irqlock-patches/remove-irqlock-2.5.27-G3

it fixes this and other related bugs as well.

Changes in -G3:

- slab.c needs to spin_unlock_no_resched(), instead of spin_unlock(). (It
also has to check for preemption in the right spot.) This should fix
the memory corruption.

- irq_exit() needs to run softirqs if interrupts not active - in the
previous patch it ran them when preempt_count() was 0, which is
incorrect.

- spinlock macros are updated to enable preemption after enabling
interrupts. Besides avoiding false positive warnings, this also

- fork.c has to call scheduler_tick() with preemption disabled -
otherwise scheduler_tick()'s spin_unlock can preempt!

- irqs_disabled() macro introduced.

- [ all other local_irq_enable() or sti instances conditional on
CONFIG_DEBUG_IRQ_SCHEDULE are to fix false positive warnings. ]

Changes in -G0:

- fix buggy in_softirq(). Fortunately the bug made the test broader,
which didnt result in algorithmical breakage, just suboptimal
performance.

- move do_softirq() processing into irq_exit() => this also fixes the
softirq processing bugs present in apic.c IRQ handlers that did not
test for softirqs after irq_exit().

- simplify local_bh_enable().

Changes in -F9:

- replace all instances of:

local_save_flags(flags);
local_irq_disable();

with the shorter form of:

local_irq_save(flags);

about 30 files are affected by this change.

Changes in -F8:

- preempt/hardirq/softirq count separation, cleanups.

- skbuff.c fix.

- use irq_count() in scheduler_tick()

Changes in -F3:

- the entry.S cleanups/speedups by Oleg Nesterov.

- a rather critical synchronize_irq() bugfix: if a driver frees an
interrupt that is still being probed then synchronize_irq() locks up.
This bug has caused a spurious boot-lockup on one of my testsystems,
ifconfig would lock up trying to close eth0.

- remove duplicate definitions from asm-i386/system.h, this fixes
compiler warnings.

Ingo

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