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Revision 774a1221e862b343388347bac9b318767336b20b authored by Tejun Heo on 16 January 2013, 02:52:51 UTC, committed by Linus Torvalds on 16 January 2013, 17:05:33 UTC 
module, async: async_synchronize_full() on module init iff async is used
 
If the default iosched is built as module, the kernel may deadlock
while trying to load the iosched module on device probe if the probing
was running off async.  This is because async_synchronize_full() at
the end of module init ends up waiting for the async job which
initiated the module loading.

 async A				modprobe

 1. finds a device
 2. registers the block device
 3. request_module(default iosched)
					4. modprobe in userland
					5. load and init module
					6. async_synchronize_full()

Async A waits for modprobe to finish in request_module() and modprobe
waits for async A to finish in async_synchronize_full().

Because there's no easy to track dependency once control goes out to
userland, implementing properly nested flushing is difficult.  For
now, make module init perform async_synchronize_full() iff module init
has queued async jobs as suggested by Linus.

This avoids the described deadlock because iosched module doesn't use
async and thus wouldn't invoke async_synchronize_full().  This is
hacky and incomplete.  It will deadlock if async module loading nests;
however, this works around the known problem case and seems to be the
best of bad options.

For more details, please refer to the following thread.

  http://thread.gmane.org/gmane.linux.kernel/1420814

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Alex Riesen <raa.lkml@gmail.com>
Tested-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Alex Riesen <raa.lkml@gmail.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Raw File
Makefile 
include scripts/Makefile.include

help:
	@echo 'Possible targets:'
	@echo ''
	@echo '  cpupower   - a tool for all things x86 CPU power'
	@echo '  firewire   - the userspace part of nosy, an IEEE-1394 traffic sniffer'
	@echo '  lguest     - a minimal 32-bit x86 hypervisor'
	@echo '  perf       - Linux performance measurement and analysis tool'
	@echo '  selftests  - various kernel selftests'
	@echo '  turbostat  - Intel CPU idle stats and freq reporting tool'
	@echo '  usb        - USB testing tools'
	@echo '  virtio     - vhost test module'
	@echo '  vm         - misc vm tools'
	@echo '  x86_energy_perf_policy - Intel energy policy tool'
	@echo ''
	@echo 'You can do:'
	@echo ' $$ make -C tools/<tool>_install'
	@echo ''
	@echo '  from the kernel command line to build and install one of'
	@echo '  the tools above'
	@echo ''
	@echo '  $$ make tools/install'
	@echo ''
	@echo '  installs all tools.'
	@echo ''
	@echo 'Cleaning targets:'
	@echo ''
	@echo '  all of the above with the "_clean" string appended cleans'
	@echo '    the respective build directory.'
	@echo '  clean: a summary clean target to clean _all_ folders'

cpupower: FORCE
	$(call descend,power/$@)

firewire lguest perf usb virtio vm: FORCE
	$(call descend,$@)

selftests: FORCE
	$(call descend,testing/$@)

turbostat x86_energy_perf_policy: FORCE
	$(call descend,power/x86/$@)

cpupower_install:
	$(call descend,power/$(@:_install=),install)

firewire_install lguest_install perf_install usb_install virtio_install vm_install:
	$(call descend,$(@:_install=),install)

selftests_install:
	$(call descend,testing/$(@:_clean=),install)

turbostat_install x86_energy_perf_policy_install:
	$(call descend,power/x86/$(@:_install=),install)

install: cpupower_install firewire_install lguest_install perf_install \
		selftests_install turbostat_install usb_install virtio_install \
		vm_install x86_energy_perf_policy_install

cpupower_clean:
	$(call descend,power/cpupower,clean)

firewire_clean lguest_clean perf_clean usb_clean virtio_clean vm_clean:
	$(call descend,$(@:_clean=),clean)

selftests_clean:
	$(call descend,testing/$(@:_clean=),clean)

turbostat_clean x86_energy_perf_policy_clean:
	$(call descend,power/x86/$(@:_clean=),clean)

clean: cpupower_clean firewire_clean lguest_clean perf_clean selftests_clean \
		turbostat_clean usb_clean virtio_clean vm_clean \
		x86_energy_perf_policy_clean

.PHONY: FORCE
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