minor GEOM disk API change coming
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Hi folks,
I have attached some patches that fix an object lifetime issue between CAM and GEOM. Fixing the bug required adding a callback to the GEOM disk code, and adding a callback that a GEOM class can register to get notified when a provider is destroyed. The probable commit message is below. If I don't hear any objections, I will commit it on Friday, June 22nd. ============ Fix a bug which causes a panic in daopen(). The panic is caused by a da(4) instance going away while GEOM is still probing it. In this case, the GEOM disk class instance has been created by disk_create(), and the taste of the disk is queued in the GEOM event queue. While that event is queued, the da(4) instance goes away. When the open call comes into the da(4) driver, it dereferences the freed (but non-NULL) peripheral pointer provided by GEOM, which results in a panic. The solution is to add a callback to the GEOM disk code that is called when all of its resources are cleaned up. This is implemented inside GEOM by adding an optional callback that is called when all consumers have detached from a provider, and the provider is about to be deleted. scsi_cd.c, scsi_da.c: In the register routine for the cd(4) and da(4) routines, acquire a reference to the CAM peripheral instance just before we call disk_create(). Use the new GEOM disk d_gone() callback to register a callback (dadiskgonecb()/cddiskgonecb()) that decrements the peripheral reference count once GEOM has finished cleaning up its resources. In the cd(4) driver, clean up open and close behavior slightly. GEOM makes sure we only get one open() and one close call, so there is no need to set an open flag and decrement the reference count if we are not the first open. In the cd(4) driver, use cam_periph_release_locked() in a couple of error scenarios to avoid extra mutex calls. geom.h: Add a new, optional, providergone callback that is called when a provider is about to be deleted. geom_disk.h: Add a new d_gone() callback to the GEOM disk interface. Bump the DISK_VERSION to version 2. This probably should have been done after a couple of previous changes, especially the addition of the d_getattr() callback. geom_disk.c: Add a providergone callback for the disk class, g_disk_providergone(), that calls the user's d_gone() callback if it exists. Bump the DISK_VERSION to 2. geom_subr.c: In g_destroy_provider(), call the providergone callback if it has been provided. In g_new_geomf(), propagate the class's providergone callback to the new geom instance. disk.9: Update the disk(9) man page to include information on the new d_gone() callback, as well as the previously added d_getattr() callback, d_descr field, and HBA PCI ID fields. ============ Ken -- Kenneth Merry [hidden email] _______________________________________________ [hidden email] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "[hidden email]" |
cam_geom_callback.20120620.1.txt (15K)
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On 21.06.2012 08:29, Kenneth D. Merry wrote:
> Fix a bug which causes a panic in daopen(). The panic is caused by > a da(4) instance going away while GEOM is still probing it. > > In this case, the GEOM disk class instance has been created by > disk_create(), and the taste of the disk is queued in the GEOM > event queue. > > While that event is queued, the da(4) instance goes away. When the > open call comes into the da(4) driver, it dereferences the freed > (but non-NULL) peripheral pointer provided by GEOM, which results > in a panic. this callback will be less useful for other classes. Does g_cancel_event() cannot help you prevent tasting? -- WBR, Andrey V. Elsukov |
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On 21.06.2012 20:48, Kenneth D. Merry wrote:
>>> In this case, the GEOM disk class instance has been created by >>> disk_create(), and the taste of the disk is queued in the GEOM >>> event queue. >>> >>> While that event is queued, the da(4) instance goes away. When the >>> open call comes into the da(4) driver, it dereferences the freed >>> (but non-NULL) peripheral pointer provided by GEOM, which results >>> in a panic. >> >> I think this situation is very specific for the GEOM_DISK class, and >> this callback will be less useful for other classes. >> Does g_cancel_event() cannot help you prevent tasting? > > Calling g_cancel_event(), for instance from disk_gone(), would not > completely close the race condition. It can't cancel an event that is > already in progress, and it is possible for the peripheral to go away while > the event is marked in progress but before the taste gets far enough into > daopen() to acquire a reference to the peripheral. periph and release it when g_destroy_provider finished. What if you will queue some custom event from the disk_gone() that will call cddiskgonecb()? Does it close the race? This event will be executed after the taste completes. -- WBR, Andrey V. Elsukov |
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