On Wed, Jun 7, 2017 at 8:41 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Tue, Jun 6, 2017 at 9:54 PM, Lv Zheng <lv.zheng@xxxxxxxxx> wrote: >> Considering this case: >> 1. A program opens a sysfs table file 65535 times, it can increase >> validation_count and first increment cause the table to be mapped: >> validation_count = 65535 >> 2. AML execution causes "Load" to be executed on the same table, this time >> it cannot increase validation_count, so validation_count remains: >> validation_count = 65535 >> 3. The program closes sysfs table file 65535 times, it can decrease >> validation_count and the last decrement cause the table to be unmapped: >> validation_count = 0 >> 4. AML code still accessing the loaded table, kernel crash can be observed. >> >> This is because orginally ACPICA doesn't support unmapping tables during >> OS late stage. So the current code only allows unmapping tables during OS >> early stage, and for late stage, no acpi_put_table() clones should be >> invoked, especially cases that can trigger frequent invocations of >> acpi_get_table()/acpi_put_table() are forbidden: >> 1. sysfs table accesses >> 2. dynamic Load/Unload opcode executions >> 3. acpi_load_table() >> 4. etc. >> Such frequent acpi_put_table() balance changes have to be done altogether. >> >> This philosophy is not convenient for Linux driver writers. Since the API >> is just there, developers will start to use acpi_put_table() during late >> stage. So we need to consider a better mechanism to allow them to safely >> invoke acpi_put_table(). >> >> This patch provides such a mechanism by adding a validation_count >> threashold. When it is reached, the validation_count can no longer be >> incremented/decremented to invalidate the table descriptor (means >> preventing table unmappings) so that acpi_put_table() balance changes can be >> done independently to each others. >> >> Note: code added in acpi_tb_put_table() is actually a no-op but changes the >> warning message into a warning once message. Lv Zheng. >> > > This still seems to be unnecessary gymnastics to keep the validation > count around and make it work for random drivers. Well, I'm not sure I agree here. If we can make it work at one point, it should not be too hard to maintain that status. Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
